This invention relates to a control system for steering an earth construction machine and, more particularly, to a system for operating both the front and rear wheels or tracks of the machine from independent sensors in an automatic mode and for operating the front wheel or track from a sensor and the rear wheel or track dependent upon the front wheel or track in a slave mode.
In the building of today's modern highways and streets, automated machinery has been utilized to more efficiently and economically construct these roads and to reduce the wear and tear on the machine operator. A wide variety of machines have been introduced to build today's modern highways and streets. For example, graders and land levelers have been used to cut a road bed into the earth having the proper slope and depth to meet highway specifications. Once the roadbed has been prepared, the curbs and gutters are put in typically by slip-form paving machines. After the curbs and gutters are in, pavers then pave the highway or street with asphalt or concrete.
Prior to automation, the roadways were cut and the pavement was laid visually to meet the pre-established specifications. As can be seen, the manual control for the machinery was slow and placed a considerable strain on the machine operator. In order to increase efficiency, automatic controls were developed for controlling these machines in grade, slope and steering axes. A stringline is now provided as a reference for both the grade and steering axes. Specifically, a grade sensor in such systems senses the deviation between the working tool of the machine, a blade of a grader or a screed of a paver, from the stringline to adjust the proper depth of the roadbed being cut or the pavement being laid. In addition, a steering sensor is referenced from the same stringline to automatically steer the vehicle. A slope sensor, usually in the form of a pendulum, is used to control the slope of the working tool to provide the proper slope for curves and crowns of the roadbed. This automatic machinery freed the operator from the drudgery of taking an active part in the control loop and allowed him a supervisory function as overseer of system operation, increasing his efficiency and reducing his wear and tear.
Usually, but not always, these earth construction machines are driven by caterpillar tracks. When the machine is supported by only two tracks, the machine is steered by adjusting the relative speed of the two tracks. For example, one track was braked, allowing the other track to rotate the vehicle to the proper heading. Machines were also introduced having four such tracks, one at each corner of the machine. These tracks could be rotated as are automobile wheels to control the heading of the machine. Some machines have only three tracks, two on one side of the machine and one on the other side of the machine. In such cases, the two tracks on the first side of the machine are steered by rotating them as are car wheels and the third track on the opposite side of the machine is not.
Although the instant invention is useful in controlling many types of machines, it is particularly useful in controlling three track machines where the tracks on one side of the machine are steerable and the track on the other side is not. In such machines, it has been found that if, in order to steer the machine, the tracks are rotated in the same direction, for example both clockwise, the track which is not steerable is then dragged or pushed over the travelled surface thus causing it to skid. Therefore, if such a machine is making a right-hand turn and its steerable tracks are on the left side of the machine, the front track is rotated clockwise and the rear track is rotated counter-clockwise to drive the machine in a manner not to skid the third track over the ground. However, a difficulty has been observed when this type of control is performed on a hillside. If the tracks are rotated in opposite directions, one track may be driving the machine uphill and the other track may be driving the machine downhill causing a yaw in the machine as it is driven along the hillside.